Electron discharge device



Dec- 31, 1957 `J. FElNsTElN 2,818,528

ELECTRON DISCHARGE DEVICE Filed Sept. l'?, 1956 United States Patent D2,818,528 ELECTRON DISCHARGE DEVICE Joseph Feinstein, Livingston, N. J.,assigner to Bell Telephone Laboratories, Incorporated, New York, N. Y.,a corporation of New York Application September 17,- 1956, Serial No.610,172

13 Claims. (Cl. S15-39.71)

This invention relates to cathode structures for electron dischargedevices and, more particularly, to cathode structures for magnetrons.

Magnetrons, as heretofore constructed, have included composite cathodestructures which utilize the phenomenon of secondary emission for theproduction of the major part of 'the anode-cathode electron stream.

It is an object of the present invention t-o improve composite magnetroncathode structures utilizing this phenomnon.

Another object of this invention is to provide an improved electrondischarge device and particularly an improved cathode structure for anelectron discharge device.

These and other objects of the present invention are realized in aspecific illustrative embodiment of the present invention wherein in amagnetron a composite cathode structure is positioned within and alongthe axis of the central cavity of a metallic anode block. The cathodestructure or assembly comprises three concentric and coaxial cylindricalmembers the middle one of which is a directly or indirectly heatedcathode capable of thermionic emission. The outermost cylindrical memberor cold cathode is formed of a material which is a good secondaryemitter, and has slots therein to permit thermionically emittedelectrons to pass therethorough. The innermost cylinder or sleeve is ofa magnetic material and is designed to facilitate the escape ofelectrons from the intercathode region in the presence of the axialmagnetic field present in all magnetrons.

Thus, a feature of this invention is a magnetron comprising an anode, asecondarily emissive cathode within the anode, a thermionic cathodewithin the secondarily emissive cathode, the anode and the cathodeshaving a common axis, a member for producing a magnetic ield parallel tothe axis, and an element for shunting the tield from the inter-cathoderegion.

Another feature of this invention is a magnetron cathode structurehaving an inner directly or indirectly heated cathode, an outer slottedsecondarily emissive cathode, and a sleeve of a magnetic material, thecathodes being coaxially and concentrically positioned and the magneticsleeve being within the inner cathode.

A complete understanding of this invention together with the above notedand other features thereof may be gained from consideration of thefollowing detailed description and accompanying drawing, in which:

Fig. 1 shows in cross section a plan view of a magnetron employing acathode constructed in accordance with this invention; and

Fig. 2 is an elevation view the device of Fig. 1.

Referring to Fig. 1, there is shown a high frequency generating deviceof the magnetron type having therein a specic illustrative embodiment ofthe present invention. A cylindrical envelope 12 contains therein acomposite cathode structure 11 which comprises a hollow slottedcylindrical non-thermionic -or cold cathode 13 located along the axis ofthe magnetron and a similarly positioned in section of the cathode ofinner cathode 14, to generate the power output for which thermionic, orhot, electron emissive cathode 14. The emissive cathode 14 may bedirectly heated, but it is shown in Figs. l and 2 as being heated by aheater element 15 which is longitudinally disposed along the axis of thestructure 11. A cylindrical element 16 made of a mag.- netic material isadvantageously positioned between the heater element and the thermioniccathode.. Surrounding the cathode structure 11 and also positionedwithin the envelope 12 is an anode member 17 having therein `a pluralityof cavity resonators 20.

A eld coil 19, which may o1' may not employ iron to aid its magneticeifect, surrounds the envelope 12 and functions to produce an intenseand constant magnetic field whose flux lines run through the envelope 12in a direction substantially parallel to the axis of the compositevcathode structure 11.

ln Fig. 2 the relative positioning of the heater 15, the

magnetic sleeve 16, the thermionic cathode 14 and the sec- -ondarilyemissive cathode 13 is clearly shown. Also shown in Fig. 2 are spacersand insulators 21, advantageously made of a ceramic material, whichareused to form part of the supporting structure for the herein describedcomposite cathode 11.

The cathode 13 is made of a material which will emit suicient electronswhen it is bombarded with a small number of primary electrons, i. e.,electr-ons from the the device 10 is designed. The cathode 13 mayadvantageously be made of beryllium copper, a good secondarily emissivematerial, i. e., one in which an impinging electron causes a pluralityof electrons to be emitted. Al ternatively, but less advantageously, thecold cathode 1 3 may be made of a suitable material coated with asecondarily emissive oxide of an earth material e. g., barium orstrontium oxide.

A cylindrical element having inside and outside diameters, respectively,of 0.160 `inch and 0.209 inch, a length of 0.25 inch and containing 6slots is used in one specic illustrative embodiment of this invention toform the outer cathode element 13 in an X band magnetron tube. Each slotmay be 0.040 inch by 0.2 inch (the longer dimension being measured in adirection parallel to the main axis inch in length. The inner cathodemay-advantageously be bonded to the magnetic sleeve Iby any suitableadhesive material. The magnetic sleeve, advantageously made ofPermendur, an alloy comprising equal parts of iron and cobalt, may be0.25 inch in length, approximately 0.020 inch thick and have an insidediameter of 0.081 inch.

The successful operation of a high power device employing a compositecathode structure depends to a'great extent upon the ability of a largenumber of primary or thermionic electrons to escape through the slots inthe secondarily emissive cathode and to strike the outer surface thereofso as to supply a large anode-cathode current.

If a proper source of potential is connectedbetween the cathodes 13 and14, and between the cathode 13 and the anode 17, an electron emittedfrom the inner cathode 14 moves in a generally circular path in theinter-cathode space, and in the anode-outer cathode region, due to theresultant action of the mutually perpendicular electric and magneticfields. The emitted electron should describe a path which will carry itthrough an outer cathode slot and then onto the surface of the outercathode to cause the emission of secondary electrons therefrom.

In specific prior art devices employing composite cathodes, the highvalue of canode-cathode magnetic ux needed for proper operation thereofalso existed in the inter-cathode space. Thus, it was found that anextremely large potential diference was required between the cathodes toobtain an electron path radius that would result in secondary emissionfrom the outer cathode. This is due to the fact that the radius of anelectrons path in an interaction space is directly proportional to thesquare root of the velocity of the electron in volts and inverselyproportional to the magnetic nx density in the interaction space.

The magnetic sleeve 16 of the present invention acts as a magnetic shuntto decrease the inter-cathode magnetic eld without affecting thestrength or configuration of the anode-cathode magnetic held. As aresult of this novel structure, electrons emitted from the inner cathodeescape through the slots of the secondarily emissive cathode and impingeupon the outer surface of the cathode 13 with a minimum of potentialdiierence connected between the two cathodes. In one specic illustrativeembodiment of the present invention, no inter-cathode potentialdifference was required, the penetration of the anode electrostatic eldthrough the outer cathode slots sutlicing to obtain starting current.Thus, embodiments of this invention provide highly reliable high poweroperation with minimum power supply requirements.

Additionally, the present invention makes possible a thermally superiorcathode design. The inner cathode structure may be designed for goodthermal insulation so that a minimum of heater power is required, whilethe slotted outer cathode may be designed for good thermal conduction.The resultant cooler operation of the outer cathode reduces arcing inthe anode-cathode main interaction space, particularly when an oxidecoated outer cathode is not used, and so provides an extremely long lifecathode. To maintain the cleanliness or" the secondarily emissivesurface it may be desirable to have it attain a temperature of several.hundred degrees centigrade in steady state operation. One would, ofcourse, then reduce the thermal conduction of the outer cathode toachieve that end.

The small potential difference connected between the cathodes ofillustrative embodiments of the present invention may be employed as ameans of controlling the start of magnetron oscillations by determiningthe time at which electrons are able to escape through the outer cathodeslots. Also, by synchronizing the small intercathode voltage with theanode-outer cathode voltage, a degree of control over the mode selectionprocess in a magnetron is realized.

It is to be understood that the above-described arrange ment isillustrative and not restrictive of the principles of the presentinvention. Other arrangements may be devised by those skilled in the artwithout departing from the spirit and scope of the invention.

What is claimed is:

1. In a magnetron cathode structure, an inner cathode,

an outer cathode, and a member of a magnetic material, said cathodesbeing coaxially and concentrically positioned, and said magnetic memberbeing within said inner cathode.

2. A magnetron cathode structure as in claim l wherein said outercathode is a cylindrical member.

3. A magnetron cathode structure as in claim l wherein said outercathode is slotted.

4. A magnetron cathode structure as in claim l wherein said outercathode is a slotted cylinder.

5. A magnetron cathode structure as in claim l wherein said outercathode is of beryllium copper.

6. A magnetron cathode structure as in claim l wherein said outercathode is a slotted cylinder made of a secondarily emissive material.

7. A magnetron cathode structure as in claim l wherein said outercathode is a slotted cylinder made of molybdenum.

8. A magnetron cathode structure as in claim l wherein said magneticmember is made of an alloy comprising equal parts of iron and cobalt.

9. In a composite cathode structure, a heater, an inner cathode, anouter cathode, and a member of a magnetic material, said cathodes beingcoaxially and concentrically positioned, said magnetic member beingwithin said inner cathode, and said heater being within said magneticmember.

l0. In combination in a magnetron, an anode block having therein acentral aperture, means for establishing magnetic flux in said aperture,and a composite cathode structure within said aperture, said cathodecomprising an inner thermionic cathode, an outer slotted cathode, saidcathodes being concentrically and coaxially positioned with respect toeach other, and means for shunting the magnetic iiux from theinter-cathode space without affecting the magnetic llux in theanode-cuter cathode space.

l1. In a magnuetron, a plurality of concentrically arranged electrodescomprising an anode, a slotted cold cathode within the anode, and athermionic cathode within the cold cathode, means for providing amagnetic eld parallel to the axis of said electrodes, and means forshunting a portion of said eld from between said cathodes comprising amember of magnetic material.

l2. A magnetron as in claim 10 wherein said shunting means is positionedwithin said thermionic cathode.

13. In a magnetron, an anode, secondarily emissive cathode means Withinsaid anode, thermionic cathode means within said secondarily emissivecathode, said anode and said cathodes having a common axis, means forproducing a magnetic held parallel to said axis, and means for shuntingsaid eld from the inter-cathode region.

No references cited.

